The possible participation of superoxide radicals in biochemical hydroxylation reactions is being investigated. Particular attention is given to the mechanism of the side-chain hydroxylation of phenylethylamine derivatives catalyzed by dopamine-beta-hydroxylase, the enzyme catalyzing the conversion of dopamine to norepinephrine. This study is significant since it may yield important information regarding the mechanism by which dopamine-beta-hydroxylase and other mono-oxygenases incorporate molecular oxygen in their substrates. The elucidation of this mechanism may help to clarify the etiology of Parkinson's disease, a neuro-enzymologic disorder in which there is a specific deficit of nigro-striatal dopamine. The identity of the active oxygen utilized by dopamine-beta-hydroxylase will be investigated by examining the effect of selected scavengers of superoxide radicals on enzyme activity, the reactivities of the oxidized and reduced enzyme with the superoxide radical by means of pulse radiolysis and stopped-flow photolysis, and the ability of the superoxide radical to support enzymatic hydroxylation in the absence of other reducing agents. Enzyme assays and product analyses utilize high pressure liquid chromatography.